My property does not update using [CallerMemberName]

Question

Admittedly I am new to wpf. But i have spent some time Googling about it all and I am stumped.

in essence i want to update my TextBlock in my UI using Binding whenever my Model values change.

So this is my Model:

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace WpfApplication1
{
    public class MyModel : INotifyPropertyChanged
    {
        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
        {
            if (Equals(storage, value))
            {
                return false;
            }

            storage = value;
            this.OnPropertyChanged(propertyName);
            return true;
        }

        public string MyField { get; set ; } 
    }
}

This is my UI:

 <Window x:Class="WpfApplication1.MainWindow"
        xmlns:viewModels="clr-namespace:WpfApplication1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        d:DataContext="{d:DesignInstance viewModels:MyModel, IsDesignTimeCreatable=True}"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <viewModels:MyModel></viewModels:MyModel>
    </Window.DataContext>
    <Grid>
        <StackPanel Orientation="Vertical">
            <TextBlock Text="{Binding  MyModel.MyField}"></TextBlock> 
          <Button Content="Click Me!" Click="Button_Click" />  
        </StackPanel>
    </Grid>
</Window>

This is my code behind:

using System.Windows;

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        public MyModel myModel = new MyModel();


        private void Button_Click(object sender, RoutedEventArgs e)
        {
            myModel.MyField = "has worked";
        }
    }
}

When i press the button the text does not change on the UI..?

Bergamot

Answer 1

The instance you create in the code behind is not the same as you assign in xaml.

Change the button click event to

private void Button_Click(object sender, RoutedEventArgs e)
{
    var model = this.DataContext as MyModel;
    model.MyField = "has worked";
}

And the binding in xaml to

<TextBlock Text="{Binding MyField}"></TextBlock>

And in your viewmodel you are not calling the notify property changed. So create a private field and modify the property as below.

private string myField;

public string MyField
{
    get { return this.myField; }
    set { this.SetProperty(ref this.myField, value); }
}

Divulge

thank you very much for such a clear and simple answer. I can accept in 3 minutes. Incidentally can the same be achieved using a static Class/Property? – Bergamot

Static class can't implement interface INotifyPropertyChanged. Use singleton instance to bind "statically" with {x:Static MyClass.Instance.Property}. – Divulge

ta for reply. i know what a singleton class is but unsure how to apply it in context to this? – Bergamot

Also, you should not be using event handlers for buttons, read about how to bind Commands from viewmodel to the ui elements. – Divulge

re: command thanks. i was going to look at that next :) why are event handlers not preferred then? – Bergamot